Substituent effect study on 13Cβ SCS of styrene series. A Yukawa-Tsuno model and Reynolds dual substituent parameter model investigation

The Yukawa–Tsuno (Y–T) and Reynolds dual substituent parameter (DSP) models have been used to model 13C substituent chemical shift (SCS) of the Cβ atom of 19 series of para‐substituted styrenes (X‐C6H4CRCYW) with variable electronic and structural demands in the side‐chain. The best fit of the Y–T model was better than that of the Reynolds DSP model for most of the studied series. A high correlation was found between the ρ value of the Y–T model and ρF value of the Reynolds DSP model. The ρ value, which reflects the sensitivity of 13Cβ SCS to the substituent field effect, was found to be influenced by the group W on the Cβ atom. A W group that enhances the para‐substituent π‐polarization of the side‐chain has a higher ρ value than its counterpart W groups that induce counter π‐polarization in the side‐chain. The series with W in an E‐configuration to the aryl ring has higher ρ value than corresponding Z series. A lower ρ value is observed when W induces a counter π‐polarization of the side‐chain (as with NO2 and COMe) or when the R substituent imposes a 65° dihedral angle between the side‐chain and the para‐substituted benzene ring (as with t‐Bu). When the W group is a heterocyclic ring, the closer the heteroatom is to Cβ, the lower the ρ value is due to the greater counter π‐polarization. The two components of the substituent effect on 13C β SCS, namely the field effect and resonance effect, behave inversely. The resonance demand (r+ value) increases, as the Y and/or W groups become more electron‐withdrawing (EW). The series with W as a hetrocyclic ring develop negative charge at the carbon atom of the hetrocyclic ring adjacent to Cβ (and to which the styryl moiety is attached) and has a lower r+ value than those which fail to do so. The lowest r+ value was for those series with a 65° dihedral angle. Copyright © 2007 John Wiley & Sons, Ltd. Y–T modeling of 13Cβ SCS gave better best fit results than Reynolds DSP modeling. An enhanced ρF and ρ was observed when τ‐polarization of (X) was enhanced by (W). The r+ increases as the (Y) and/or (W) groups become more electrons withdrawing. Lower r+ was observed, when (W) is heterocyclic ring which develops a negative charge at the carbon atom of the heterocyclic ring adjacent to the Cβ than those which fail to do so. Twisting the side chain by 65° reduces ρF, ρR, ρ and r+ values.